Mankind has struggled to understand and control the effects of air flow on an object introduced into the air stream since the beginning of the development of useful tools. Thrown spears, shot arrows, flying planes and golf balls all are affected by the force of the air, as is any object, as it moves through the air.
For example only, and not by way of limitation, a difficulty solved by the Applicant's invention relates to achieving accurate and repeatable arrow flight of wide cutting width fixed blade broad head tipped arrows (“broad heads”). The current standard hunting broad heads are designed with the intention of killing via penetration of the chest cavity. This is a very efficient and humane manner of killing most any animal on the face of the earth. Nonetheless, exceptions exist such as with large game birds. For example only and not by way of limitation, the wild turkey is a difficult game bird. A turkey has disadvantages associated with aiming for the chest area and vital internal organs in this region of the bird's body. To begin with a turkey's vital target area is very small. Further, they do not have a significant volume of blood. Additionally, feathers prevent blood sign from reaching the ground and further aid in coagulation of the blood in the wound channel. Additionally, turkeys can run very fast and/or fly after even a mortally placed shot. In fact, common practice after a broad head strikes the turkey is for the archer to immediately jump up and run after the mortally hit game bird in an attempt to physically prevent its escape. This effort brings a hunter in close contact with razor sharp blades as the bird thrashes about. Additionally, chest shots generate significant waste of edible portions of the bird.
While broad heads are useful hunting tools, they would be even more useful if they could be accurately delivered to an area of the animal, such as to the head/neck region, which would cause instant death. This eliminates all the negative aspects as described when hunting turkeys or other game bird with broad heads meant to hit the chest cavity/body of the bird. Unfortunately, the evolution of the broad head has provided no significant changes in design or shape other than those advantages and efficiencies derived from newer materials and better machining techniques for fixed blade broad heads. With the advent and availability of improved materials, the bow for delivering the arrow has also improved considerably. Compound bows are much more efficient than traditional equipment and result in the capability to launch arrows at considerably higher velocities. Unfortunately, these higher velocities introduce significant aerodynamic problems in maintaining accurate arrow flight with a broad head attached. This unwanted resultant inaccurate arrow flight has been termed “steering effect”. Prior art attempts to minimize this steering effect have taken two directions.
Currently, one solution is to stay with the traditional two, three, four or more blades rigidly affixed to the ferrule. Here, attempts to minimize the steering effect on larger diameter cutting width broad heads have focused on reducing the surface area of fixed blades in two manners. First, the blades overall cutting width has been reduced to maintain as narrow an aerodynamic profile as possible. In this case the blades are swept back from the tip like wings on a fighter aircraft. Additionally, cut outs within the blade were implemented. Currently, minimum cutting widths of no less than seven-eights of an inch are permitted. Generally acceptable flight is achieved at these widths. However, the steering effect is exacerbated with increasing arrow velocities achieved with today's modern bows. Even a narrow width, swept back, blade can cause trouble in achieving repeatable accurate arrow flights due to pressure exerted by the air, up drafts, down drafts or wind as the arrow flies to its intended target.
A second prior art “solution” to eliminate the steering effect problem has been to create a broad head that has its blades closed during flight. Upon contacting the intended target, these broad heads include some form of mechanism that causes the blades to pop open on impact thus exposing lethal cutting surfaces. With no flat surfaced blades exposed during flight, the steering effect is minimized since there are no pressure differences generated on exposed blade surfaces. Several disadvantages of these so-called “mechanical” broad heads exist such as, for example only, reduced penetration of the broad head, structural weakness of the various broad head elements, and inoperability at the critical moment of contact with the game animal. Additionally, much more kinetic energy is required to achieve equal penetration compared to fixed broad head blades.
In short, maintaining strength upon impact, having large cutting widths, achieving good penetration and maintaining accurate arrow flight are the desired characteristics of a hunting arrow tipped with a broad head blade. Modern manufacturing and materials allow simple production of strong broad heads with razor sharp blades to kill with maximum efficiency upon contact with the hunted animal. Flight, however, is the operative word and repeatable accurate flight is the desired goal. The undesirable steering effect on the current broad head blades, however, results in a loss of control in the flight of an arrow that results in poor accuracy.
Aerodynamics is the study of gases in motion. The principal application of aerodynamics is in the design of aircraft and air is the gas with which the science is most concerned. Understanding arrow flight, or the flight of any other object, requires an understanding of how a wing works to lift an airplane. An arrow with a broad head attached is influenced by the same principles as described by Newton's laws as those that describe the function of an airplane wing and the generation of lift.
The shape of the air foil (wing) is a very important part of lift. Most airfoils today have camber, meaning they have curved upper surfaces and flatter lower surfaces. These airfoils generate lift even when the air flow is horizontal (flat). Symmetric airfoils are airfoils wherein the upper and lower surfaces are the same length. The particles of the air stream above and below symmetric airfoils move at the exact same velocity. As a result, no lift is generated by a symmetric airfoil in horizontal flow (flat wings moving straight ahead cannot fly).
In order to generate lift with a symmetric airfoil, the airfoil must be turned (tilted) with respect to the flow of the air, so that the upper surface is “lengthened” and lower surface is “shortened”. This tilting against the air flow is called “angle of attack”. It has been shown that if the angle of attack is doubled, the lift doubles. In keeping with the above example concerning broad head tipped arrows, this is exactly why broad head tipped arrows are prone to inaccurate flight. The problem is exacerbated with larger cutting surfaces and greater distances from point of release. The most elegant aerodynamic scheme is to obtain a zero lift condition in which a zero angle of attack corresponds to zero coefficient of lift. In the case of an aircraft, lift is desirable. In the case of a broad head tipped arrow, no lift is desired.
Airplanes have a variety of control surfaces utilized during flight to control flight direction. An arrow is simply launched and travels with no additional human intervention to correct its intended flight path in the event of it moving off the desired flight path. Arrows do contain fletching (feathers or plastic vanes) on the rear portion of the arrow in attempts to impart some semblance of in flight control. This is accomplished via primarily drag forces imparted via helical or offset placement of the fletching designed to keep the arrow aligned and on track so as to accurately hit its intended target. It is intended that the fletching overcome any appreciable amount of torgue or lift applied to the exposed surfaces of the arrow. Spin is also imparted in combination with the rear drag to overcome any negative influencing aerodynamic factors between the point of release and striking of the target. Yet it is just this spin that also contributes a difference of pressure (generation of lift) to the exposed surfaces of the broad head blades (wing span) immediately upon acceleration of the arrow from the bow. This contributes immediately to inaccurate arrow flight as compared to a target tipped arrow (ie no broad head blades or any blades or “wings” at all). With no movable controls to correct the in-flight steering effect, accuracy of broad head tipped arrows is lost. By applying too much drag to control the broad head tipped arrow from experiencing the steering effect, not enough energy is retained to provide a killing shot at the hunted animal. Further, hunting distance is severely limited, making for an exceptionally more difficult successful hunting experience.
Further, the role of the air resistance must also be considered when examining the path of an object through the air. Air resistance acts to retard the forward travel of an arrow in flight, for example. Just as too much drag at the rear of the arrow adversely affects the arrow's path, too much pressure up front results in the same negative effect. A narrow width bladed broad head has less cross-sectional surface area to compress air as it travels through space. A wide width blade has increased sectional surface area to compress air as it travels through the same given space. Just as pressure differences can cause undesirable lift and unwanted steering effects, this pressure also contributes to rob the arrow of kinetic energy as it uses up more energy crossing the same given distance. Loss of velocity translates to a less flat trajectory given the arrow travels the same distance as a narrow width broad head. A flat trajectory is important in a hunting situation where an estimation of distance to the target animal adds error to the dynamics of an accurately placed lethal shot. A flatter trajectory allows for a larger margin of human error in achieving an accurately placed shot.
Considering the effect of lift, torque and pressure, it is easy to understand why it is possible to achieve repeatable and accurate flight of a target tipped arrow (no wings) versus the less accurate flight due to steering effects with a broad head tipped arrow (wings) shot from the same bow given all other factors being equal. In both cases, at the moment of transfer of the kinetic energy stored in the limbs of the bow to the movement of the arrow from rest to some given velocity, the shaft flexes in response to this absorption and transfer of energy. With the streamlined shape of the standard target tipped arrow, with no planner surfaces to leverage against, coupled with the minimal frontal arrow surface area to compress and deflect air of any appreciable magnitude in a concerted direction, minimal lift is created thus providing minimal deflection of the target tipped arrow from its intended path. As a result, accurate arrow flight is achieved quite easily due to the minimal impact of lift, torque and pressure on the intended flight path of a target tipped arrow.
On the other hand, in the case of the broad head tipped arrow, it is easy to understand why it experiences unwanted steering effect. Again, at the moment of transfer of the kinetic energy stored in the limbs of the bow to the movement of the arrow from rest to some given velocity, the shaft flexes in response to this absorption. In this case however, this deflection immediately changes the angle of attack of the broad head blade surfaces from the desired zero angle of attack to some greater value. The combination of the forward velocity of the arrow passing through the medium of the air, its changing angles of attack due to each oscillation of the arrow shaft as it travels away from the bow, rotation of the broad head blades creating pressure differences on each side of the blades, and lift created in differing amounts, all cause the adverse steering effects on the broad head tipped arrow. When other contributing factors such as updrafts, downdrafts, and cross winds are coupled with this steering effect on broad head tipped arrows, the magnitude of deflection from the intended target is further amplified and even more chaotic resulting in a greater degree of inaccuracy and less likelihood of repeatedly striking a target in the same place.